This invention relates to a thermoplastic resin composition in which compatibility of a polyolefin type resin and a saturated polyester is improved, a balance between heat resistance and mechanical strength is excellent and also molding processability, water absorption resistance and appearance of a molded product are excellent, and a process for preparing a modified polyolefin type resin having excellent adhesiveness, coatability, printability and polymer-modifying property, by subjecting a polyolefin type resin to graft polymerization with a monomer having a polar functional group.
A polyolefin type resin represented by a polypropylene resin and a polyethylene resin has low specific gravity, a low price and also excellent mechanical strength, luster, transparency, moldability, water absorption resistance and chemical resistance so that it has been widely used alone or as one component of a polymer blend for preparing various molded products. However, the polyolefin type resin has a non-polar molecular structure so that its affinity for other substance is poor, whereby various characteristics such as adhesiveness, coatability, printability, antistatic property and compatibility in a polymer blend are extremely poor. Also, heat resistance of the polyolefin type resin is not so high, and such a drawback is an obstacle to its use as an engineering plastics.
On the other hand, a saturated polyester represented by polybutylene terephthalate and polyethylene terephthalate has been recognized as an engineering plastics having excellent heat resistance, chemical resistance and electrical characteristics. However, the saturated polyester has a drawback that water absorption resistance is poor so that water is easily absorbed into the saturated polyester during molding processing such as injection molding and extrusion molding and especially under conditions of high temperature and high humidity, whereby physical properties such as impact resistance are lowered.
As one of attempts to cope with the case where desired various properties cannot be satisfied sufficiently by one resin material, there has been frequently used a technique of compensating insufficient properties by mixing it with other resin material. If a composition having both good properties of the saturated polyester and the polyolefin type resin and compensating undesirable properties each other can be obtained by such a technique, an excellent resin material having a wide application field can be provided.
However, the saturated polyester and the polyolefin type resin do not have compatibility with and affinity for each other so that when the both components are simply mixed, adhesiveness at the interface of this two phase structure is not good. Therefore, the interface of a molded product obtained becomes a defective portion to lower mechanical strength. Also, these two phases can hardly take uniform and fine dispersion forms so that when shear stress is applied during molding processing such as injection molding, laminar peeling (delamination) is easily caused.
In order to solve the above problems, it has been proposed to blend into a saturated polyester, a modified polyolefin type resin obtained by introducing functional groups which react with the saturated polyester into a polyolefin type resin having poor reactivity.
For example, a resin composition comprising an unsaturated carboxylic acid-modified polypropylene resin and a saturated polyester has been disclosed in Japanese Provisional Patent Publication No. 74168/1981. However, the unsaturated carboxylic acid-modified polypropylene resin and the saturated polyester have insufficient dispersibilities so that laminar peeling is easily caused, whereby a satisfactory resin composition for practical use cannot be obtained. Further, resin compositions each comprising an epoxy-modified polyolefin type resin and a saturated polyester have been disclosed in Japanese Provisional Patent Publications No. 60746/1986 and No. 213352/1989. When these compositions are used, compatibility can be improved to some extent, but the compositions are partly gelled during melting and kneading by an extruder or the like to lower fluidity, whereby there are problems that extruded strand cannot be drawn stably and surface appearance of an extrusion molded product is worsened.
Further, the present inventors have studied a composition comprising a polyolefin type resin modified with a hydroxyl group-containing .alpha.,.beta.-unsaturated carboxylate, and a saturated polyester resin (Japanese Patent Application No. 235862/1992), but the addition amount of hydroxyl groups is not sufficient so that further improvement is required.
In order to remove above drawbacks, there have been attempted a number of methods for preparing modified resins by subjecting a polyolefin type resin to graft polymerization with a polymerizable monomer having a polar functional group. For example, a method of modifying a polyolefin type resin by using a methacrylate or acrylate having a hydroxyl group for the purpose of improving adhesiveness or coatability has been disclosed in Japanese Provisional Patent Publication No. 160449/1979, but the addition amount of hydroxyl groups is not sufficient for practical use particularly as a compatibilizer. There has been known a method of using a radical polymerization initiator having high hydrogen-abstracting ability in order to increase the graft polymerization amount of a monomer having a hydroxyl group to a main polymer chain. In this method, molecular chain cleavage of the main polymer chain is also increased to cause problems such as lowering of physical properties.
As a method of preventing molecular chain cleavage of a main polymer chain, there has been generally used a method of preventing deterioration caused by heat or oxygen in a reaction system. As a specific example thereof, there may be mentioned a method of carrying out a reaction under inert gas or under reduced pressure, but it cannot be said that this method has a sufficient effect.
That is, there has not yet been proposed a means which can simultaneously achieve two tasks of increasing the graft polymerization amount of a monomer having a hydroxyl group and preventing molecular chain cleavage.
Further, as a result of precisely analyzing a modified polyolefin resin obtained by subjecting a polyolefin type resin to graft polymerization with a methacrylate type monomer having a hydroxyl group (e.g. 2-hydroxyethyl methacrylate) by a melting reaction, it has been found that there is a problem that a number of ester portions to which a hydroxyl group is bonded are lost by hydrolysis or the like. That is, a task of increasing a hydroxyl group-remaining percentage is also one of the important tasks of this kind of modifying method, and a means for achieving the task has been demanded.